Hypoflexid function in the “trenchant heel” of carnassial teeth, with comments on talonid evolution

The carnassial teeth of Carnivora and Dasyuromorphia are characterized by the enlargement of the carnassial blades and reduction of crushing structures. In some species, the highly carnassialized teeth exhibit a unicuspid talonid with only the hypoconid present (“trenchant heel”). This condition is similar to that seen in the molars of pretribosphenic cladotherians such as Dryolestida, with a single talonid cusp and hypoflexid groove. Tooth wear and reconstruction of the power stroke show that the hypoflexid of the trenchant heel occludes with the paracone of the distal upper antagonist, providing a cutting and guiding function during the power stroke, and maintaining a uniform inclination of the tooth movement up to the point of centric occlusion. In case of the Dasyuromorphia, this occlusal relationship is most pronounced between the distal molars (M4/m4), whereas in the Carnivora it occurs between the upper and lower mesial molars (M1/m1). The occurrence of distal hypoflexid-like grooves is a recurring trend in mammal evolution, before and after the evolution of tribosphenic molars with multicuspid talonid.